This invention concerns a double super heterodyne receiver.
In general, because carrier waves are amplified by two narrow-band amplification circuits, consisting of a radio frequency amplification circuit and an intermediate frequency amplification circuit, super heterodyne receivers are characterized by their ability to stabilize carrier wave amplification to the detector and by suitable selectivity.
However, FM receivers involve the following. Carrier waves reaching the FM receiver are obstructed by noise within the passband of the selective amplifier up to the FM detector. That appears as so-called FM noise in the output of the FM detector. However, this noise diminishes in inverse proportion to the electrical power of the carrier wave reaching the receiver. In other words, it is proportional to the signal-to-noise (SN) ratio. Accordingly, since the FM noise is below the residual noise of the FM detector at a sufficient carrier wave level, the limit of the SN ratio of the receiver demodulation output would be determined by the ratio of the FM detector output to the residual noise of the detector. The residual noise of the FM detector varies with the design, and improvement must be made in the products which constitute the FM receiver in order to reduce this noise. The SN ratio of the FM detector output can also be improved by improving the detection efficiency, however.
With this in mind, we will next investigate the properties of the FM detector from the demodulation theory of FM waves. In FM wave detection, FM waves are imposed on a circuit in which the amplitude or phase changes linearly in relation to the deviation in frequency and detection is carried out by detection of the phase or amplitude deviation of the output signal. FM detectors using the former method include twin tuning detectors, Forster Seeley detectors and radio detectors, while FM detectors using the latter method include quadrature detectors and pulse count type detectors.
We will first consider the properties of an FM detector based on the first method.
When the frequency modulated wave is modulated by the modulating wave S(t) expressed by the following formula (1), the frequency modulated wave i(t) is represented by formula (2): EQU S(t)=I.sub.s cos pt (1) EQU i(t)=I.sub.O sin (.omega..sub.O t+m.sub.f sin pt). (2)
In formula (2), however, I.sub.O is the amplitude of the carrier wave, .omega..sub.O is the angular frequency of the carrier wave and m.sub.f is the modulation index. The modulation index m.sub.f is represented by the following formula when the maximum angular frequency deviation is taken as .DELTA..omega. and the constant is K. EQU m.sub.f =(.DELTA..omega./p)=(KI.sub.s /p.) (3)
When a frequency modulated wave i(t) is imposed on a circuit whose output frequency changes linearly in relation to the frequency, such as a differential circuit, the output i'(t) would be represented as follows: EQU i'(t)=I.sub.O .omega..sub.O (1+m.sub.a cos pt) cos (.omega..sub.O t+m.sub.f sin pt). (4)
The following is also true: EQU m.sub.a =.DELTA..omega./.omega..sub.O. (5)
Specifically, if the frequency modulated wave i(t) passes through the differential circuit, the output will be a amplitude modulated wave with modulation index m.sub.a and envelope cos pt. Accordingly, demodulation of the frequency modulated wave is possible if envelope line detection of this output i'(t) is conducted. If we assume that the maximum angular frequency deviation .DELTA..omega. is constant, then the degree of modulation m.sub.a increases as the angular frequency of the carrier wave .omega..sub.O decreases.
Next we will consider the properties of an FM detector based on the second method. When the frequency modulated wave i(t), represented by the same formula (2) as in the previous case, is imposed on a circuit in which the output phase deviates linearly in relation to the frequency deviation, such as a phase shift circuit, the following formula develops, assuming the central phase angle of the carrier wave angular frequency .omega..sub.O to be .theta..sub.O, the deviation of the output phase angle in relation to frequency change to be .tau. and the phase differential in relation to the input of the phase shift circuit output to be .theta.(t). ##EQU1## .tau. is called the delay time of the phase shift circuit. Accordingly, demodulation of the frequency modulated wave would be possible if an electronic circuit were designed in which the voltage were proportional to the phase difference between the output side and the input side of the phase shift circuit expressed in formula (6). If the maximum angular frequency deviation .DELTA..omega. were assumed to be constant, the phase difference would increase directly with the delay time .tau.. Thus, a large demodulation signal would be achieved. In addition, the delay time .tau. would be greater in relation to the phase shift circuit of same structure as the angular frequency of the carrier wave .omega..sub.O became smaller. Using a pulse counter detector as an example, the pulse width would be greater the smaller the carrier wave frequency. This is the selection of a large delay time .tau..
As indicated above, if the maximum angular frequency deviation .DELTA..omega. is constant, the FM detector will have the property of an improved detection efficiency the smaller the carrier wave angle frequency .omega..sub.O. Accordingly, the use of the super heterodyne method in the FM receiver would result in higher detection efficiency, improved SN ratio and the ready achievement of a better SN ratio the lower the intermediate frequency selected.
However, the intermediate frequency amplification circuit in the FM receiver ensures the necessary occupied bandwidth in FM broadcast and reception and, since selective amplification with a sufficient bandwidth in relation to fluctuation of the oscillation frequency of the local oscillator must be conducted, the intermediate frequency cannot be easily lowered. The double super heterodyne method was conceived to eliminate such problems. Specifically, the first intermediate frequency is set at the frequency value so that said problems do not develop. Frequency conversion of this first intermediate frequency is repeated and a second intermediate frequency is achieved with a still lower frequency value. The detection efficiency by FM detection is improved.
However, although the double super heterodyne method solves various problems, it does give rise to new ones. Specifically, when the first intermediate frequency output or the second local oscillator output leaked out in the second intermediate frequency output, the frequency value of the higher harmonic of the second intermediate frequency approaches the values of the first intermediate frequency or of the second local oscillator output frequency and a beat component develops with frequency equal to the difference between the two. This beat component appears in the demodulation output as noise.